The simplified answer

A selective coating absorber captures the photons from the
electromagnetic spectrum (Sunlight) and gets hot. Heat trans
fer fluid (anti-freeze) is pumped through the hot absorber
therefore itself becoming hot and carrying the heat down to a
hot water cylinder. The cold water in the cylinder is heated up
to be used for baths and showers.

That is a very brief explanation, but some may prefer a more
in depth answer to the question, “How do solar panels work?”
Here is the more detailed response, and require a little physics
to understand how heat energy moves about.

The Physics

To start off, heat is measured by temperature and only flows
one way. You can’t make something hotter by placing it next
to something that is colder!
There are three ways in which heat energy can be trans-
ferred :-
Conduction: The heat energy moves through the
material. A good thermal conductor such as metal allows heat
energy to pass quickly and easily through it. For example
place one end of an iron bar in a fire and the other end ( the
bit you’re holding!) get hot quickly because metal is a good
conductor of heat. The end of a wooden pole in the same
situation can be held for a considerable period of time be-
cause wood is a poor conductor of heat. All materials conduct
heat in varying degrees and surprisingly enough water is a
poor conductor of heat which allows hot water in a cylinder to
stay hot even with cold water below it.

Convection; here the heat energy is carried by moving
the
material
with
its
heat
energy
somewhere
else.
For example, a radiator in a room will heat the air next to it, that
air then rises and carries its heat around the room warming
the whole room. This is natural convection. Forced convection
is where the material say for example the water in a boiler is
physically moved by the pump to the radiator where the radiator
then
gets
hot.
Because
of
its
very
high
heat
capacity water is a very good medium for convecting heat from one
place to another.

Radiation: this is the direct transfer of heat by the
electromagnetic spectrum (Solar radiation). You can feel radiated
heat
coming
off
a
fire.

In the case of a solar thermal panel we are trying to heat
above the ambient temperature so conduction and convection
will work against us by taking heat from the panel to the out-
side world.

The only way you can get a higher temperature than ambient
is by radiation therefore all solar collectors rely on Solar radiation!

All the energy from the Sun comes via radiation. In the vacuum
of space there is no material to conduct and no material for
convection. The sun (at 6000 C surface temperature) is hotter
than the solar panel so the panel will get hot due to the solar
radiation. (this is the only way to get a higher temperature
than the surroundings so all solar panels need solar radiation to
get hot!)

The Terminology

The effectiveness of the collector will depend on how good it is
at absorbing radiation (Selective coating) and the heat loss to
the ambient temperature (Insulation factor) .
As the collector heats up, the amount of heat lost to ambient
will depend on the difference in temperature between the panel
and ambient.

The heat loss occurs by conduction, convection and radiation.
The higher the difference (hotter the panel) the faster the heat
is lost until the amount of heat lost is the same as the amount
of heat gained by radiation. The solar panel will then stop getting
any
hotter and this temperature
is
known
as the
stagnation
temperature.
The more power-
ful the incident
radiation
(measured in
watts/M
2
) the
more energy the
panel needs to
loose so the
higher the stagnation
temperature becomes.

Any time there is daylight there is solar radiation but the
amount of energy available will depend on the incident radiation
2
level (watts/m
). In bright sunshine there is about 1000 watts
of available heat energy for every square meter, on cloudy days
there is still solar radiation available but it will be at reduced
2
intensity (lower watts/M).

At stagnation the collector temperature is so high compared
with ambient that any heat extracted will drop the temperature
so effectively the efficiency is 0 ( 1000 watts/m2 in gives a very
high temp but correspondingly high losses so available heat out
is zero).

It therefore follows that the maximum efficiency occurs with the
collector temperature being low compared with ambient ( 1000
W/m2 in gives low losses and around 930 watts of heat out
hence the efficiency is 93%) If a solar panel is 93% efficient then it takes 93% of the available energy and turns it into heat,
on a cloudy day it is still 93% efficient but the amount of avail-
able heat is less hence the panel output is less.

Size matters! Because the solar radiation is uniform at so many
watts/M
then for any collector, if you want twice as much energy
the
collector
needs
to
be
twice
as
big!

Performance parameters

The term solar collector applies to a device that is specifi-
cally designed harvest energy from solar radiation.

Everything will get hot when exposed to solar radiation
(think of a garden hose in summer! The water can get quite
warm but as soon as you try and move that heat to somewhere
useful,
a
paddling
pool
for
example,
the
water
soon runs cold as the hose isn’t a particularly good solar panel!).

So the performance of a solar panel will depend on how
good it is at “getting hot”.

We have already touched on the ability of a solar collector
to deliver hot water depends on the
Selective coating and
Insulation factors.

The heat loss to ambient by conduction and convection can
be limited by good insulation or stopped completely by a
vacuum.

In a vacuum tube the heat loss can only be by re radiation,
as a materials temperature rises then the radiant heat
given off rises (
Black body radiation) the higher the temperature the shorter the wavelength of the radiation given
off.

For example “red hot” is a lower temperature than “white
hot” and the corresponding wavelengths of red light are
longer than the rest of the colours that make up white light.

Longer wavelengths below red (infer-red) are where radiant
heat is.

The ability of a material to absorb radiation depends on
colour, black is very good at absorbing radiation but also
good at re-radiating it (hold your hand over a material that
is black and hot you can feel the radiant heat ).
In the case of a solar collector we need a material that absorbs
radiation
effectively
over
the
whole
spectrum
but doesn’t re-radiate it, hence the term
selective coating.

This is why most solar collectors look dark blue/grey rather
than matt black.

The stagnation temperature of a vacuum tube collector will
always be higher than other collector types because the
heat loss by
conduction and convection is elim